.sctn. 1.1 Field of the Invention
The present invention concerns techniques for enhancing the resolution of characters, such as fonts for example, to be rendered on a patterned output device, such as a flat panel video monitor or an LCD video monitor for example. More specifically, the present invention concerns techniques for maintaining the width of fonts, and related challenges, so that existing formatting remains unchanged when the resolution of characters is enhanced. The present invention also concerns minimizing visually annoying variations in spacing and line weight.
.sctn. 1.2 Related Art
Before an introduction of related art, it must be understood that the related art described here is not necessarily "prior art" and that the description of such related art in this section is not to be construed as an admission that the art is "prior art", unless expressly stated.
The present invention may be used in the context of patterned output devices such as flat panel video monitors, or LCD video monitors for example. In particular, the present invention may be used as a part of processing to produce higher resolution characters, such as text for example, on patterned displays, such as LCD video monitors for example. Although the structure and operation of display devices in general, and patterned display devices, such as LCD monitors for example, in particular, are known by those skilled in the art, they are discussed in .sctn. 1.2.1 below for the reader's convenience. Then, known ways of rendering characters on such displays are discussed in .sctn. 1.2.2 below.
.sctn. 1.2.1 Display Devices
Color display devices have become the principal display devices of choice for most computer users. Color is typically displayed on a monitor by operating the display device to emit light (such as a combination of red, green, and blue light for example) which results in one or more colors being perceived by the human eye.
Although color video monitors in general, and LCD video monitors in particular, are known to those skilled in the art, they are introduced below for the reader's convenience. In .sctn. 1.2.1.1 below, cathode ray tube (or CRT) video monitors are first introduced. Then, in .sctn. 1.2.1.2 below, LCD video monitors are introduced.
.sctn. 1.2.1.1 CRT Video Monitors
Cathode ray tube (CRT) display devices include phosphor coatings which may be applied as dots in a sequence on the screen of the CRT. A different phosphor coating is normally associated with the generation of different colors, such as red, green, and blue for example. Consequently, repeated sequences of phosphor dots are defined on the screen of the video monitor. When a phosphor dot is excited by a beam of electrons, it will generate its associated color, such as red, green and blue for example.
The term "pixel" is commonly used to refer to one spot in a group of spots, such as rectangular grid of thousands of such spots for example. The spots are selectively activated to form an image on the display device. In most color CRTs, a single triad of red, green and blue phosphor dots cannot be uniquely selected. Consequently, the smallest possible pixel size will depend on the focus, alignment and bandwidth of the electron guns used to excite the phosphor dots. The light emitted from one or more triads of red, green and blue phosphor dots, in various arrangements known for CRT displays, tend to blend together giving, at a distance, the appearance of a single colored light source.
In color displays, the intensity of the light emitted from the additive primary colors (such as red, green, and blue) can be varied to achieve the appearance of almost any desired color pixel. Adding no color, i.e., emitting no light, produces a black pixel. Adding 100 percent of all three (3) colors produces a white pixel.
Having introduced color CRT video monitors, color LCD video monitors are now introduced in .sctn. 1.2.1.2 below.
.sctn. 1.2.1.2 LCD Video Monitors
Portable computing devices (also referred to generally as computing appliances or untethered computing appliances) often use liquid crystal displays (LCDs) or other flat panel display devices, instead of CRT displays. This is because flat panel displays tend to be smaller and lighter than CRT displays. In addition, flat panel displays are well suited for battery powered applications since they typically consume less power than comparably sized CRT displays. Further, LCD flat panel monitors are even becoming more popular in the desktop computing environment.
Color LCD displays are examples of display devices which distinctly address elements (referred to herein as pixel sub-components, pixel sub-elements, or simply, emitters) to represent each pixel of an image being displayed. Normally, each pixel element of a color LCD display includes three (3) non-square elements. More specifically, each pixel element may include adjacent red, green and blue (RGB) pixel sub-components. Thus, a set of RGB pixel sub-components together may define a single pixel element.
Known LCD displays generally include a series of RGB pixel sub-components which are commonly arranged to form stripes along the display. The RGB stripes normally run the entire length of the display in one direction. The resulting RGB stripes are sometimes referred to as "RGB striping". Common LCD monitors used for computer applications, which are wider than they are tall, tend to have RGB vertical stripes. Naturally, however, some LCD monitors may have RGB horizontal stripes.
FIG. 1 illustrates a known LCD screen 100 comprising pixels arranged in a plurality of rows (R1-R12) and columns (C1-C16). That is, a pixel is defined at each row-column intersection. Each pixel includes a red pixel sub-component, depicted with moderate stippling, a green component, depicted with dense stippling, and a blue component, depicted with sparse stippling. FIG. 2 illustrates the upper left hand portion of the known display 100 in greater detail. Note how each pixel element, such as, the (R2, C4) pixel element for example, comprises three (3) distinct sub-element or sub-components; a red sub-component 206, a green sub-component 207 and a blue sub-component 208. In the exemplary display illustrated, each known pixel sub-component 206, 207, 208 is 1/3, or approximately 1/3, the width of a pixel while being equal, or approximately equal, in height to the height of a pixel. Thus, when combined, the three 1/3 width, full height, pixel sub-components 206, 207, 208 define a single pixel element.
As illustrated in FIG. 1, one known arrangement of RGB pixel sub-components 206, 207, 208 form what appear to be vertical color stripes on the display 100. Accordingly, the arrangement of 1/3 width color sub-components 206, 207, 208, in the known manner illustrated in FIGS. 1 and 2, exhibit what is sometimes called "vertical striping".
In known systems, the RGB pixel sub-components are generally used as a group to generate a single colored pixel corresponding to a single sample of the image to be represented. More specifically, in known systems, luminous intensity values for all the pixel sub-components of a pixel element are generated from a single sample of the image to be rendered.
Having introduced the general structure and operation of known LCD displays, known techniques for rendering text on such LCD displays, as well as perceived shortcomings of such known techniques, are introduced in .sctn.1.2.2 below.
.sctn. 1.2.2 Rendering Text on Displays
The expression of textual information using font sets is introduced in .sctn. 1.2.2.1 below. Then, the rendering of textual information using so-called pixel precision and perceived shortcomings of doing so are introduced in .sctn. 1.2.2.2 below.
.sctn. 1.2.2.1. Font Sets
A "font" is a set of characters of the same typeface (such as Times Roman, Courier New, etc.), the same style (such as italic), the same weight (such as bold and, strictly speaking, the same size). Characters may include symbols, such as the "Parties MT", "Webdings", and "Wingdings" symbol groups found on the Word.TM. word processor from Microsoft Corporation of Redmond, Washington for example. A "typeface" is a specific named design of a set of printed characters (e.g., Helvetica Bold Oblique), that has a specified obliqueness (i.e., degree of slant) and stoke weight (i.e., line thickness). Strictly speaking, a typeface is not the same as a font, which is a specific size of a specific typeface (such as 12-point Helvetica Bold Oblique). However, since some fonts are "scalable", the terms "font" and "typeface" may sometimes be used interchangeably. A "typeface family" is a group of related typefaces. For example, the Helvetica family may include Helvetica, Helvetica Bold, Helvetica Oblique and Helvetica Bold Oblique.
Many modern computer systems use font outline technology, such as scalable fonts for example, to facilitate the rendering and display of text. TrueType.TM. fonts from Microsoft Corporation of Redmond, Washington are an example of such technology. In such systems, various font sets, such as "Times New Roman," "Onyx," "Courier New," etc. for example, may be provided. The font set normally includes a high resolution outline representation, such as a series of contours for example, for each character which may be displayed using the provided font set. The contours may be straight lines or curves for example. Curves may defined by a series of points that describe second order Bezier-splines for example. The points defining a curve are typically numbered in consecutive order. The ordering of the points may be important. For example, the character outline may be "filled" to the right of curves when the curves are followed in the direction of increasing point numbers. Thus the high resolution character outline representation may be defined by a set of points and mathematical formulas.
The point locations may be described in "font units" for example. A "font unit" may be defined as the smallest measurable unit in an "em" square, which is an imaginary square that is used to size and align glyphs (a "glyph" can be thought of as a character). FIG. 3 illustrates an "em" square 310 around a character outline 320 of the letter Q. Historically, an "em" was approximately equal to the width of a capital M. Further, historically, glyphs could not extend beyond the em square. More generally, however, the dimensions of an "em" square are those of the full body height 340 of a font plus some extra spacing. This extra spacing was provided to prevent lines of text from colliding when typeset without extra leading was used. Further, in general, portions of glyphs can extend outside of the em square. The coordinates of the points defining the lines and curves (or contours) may be positioned relative to a baseline 330 (Y coordinate=0). The portion of the character outline 320 above the baseline 330 is referred to as the "ascent" 342 of the glyph. The portion of the character outline 320 below the baseline 330 is referred to as the "descent" 344 of the glyph. Note that in some languages, such as Japanese for example, the characters sit on the baseline, with no portion of the character extending below the baseline. Each character includes two (2) points on its baseline 330--the character origin (or "CO") 332 and the concatenation point (or "CP") 334. The concatenation point 334 of one character will coincide with the character origin point 332 of a next adjacent character. The distance between the character origin 332 and the concatenation point 334 is referred to as the advance width (or "AW") 370. The stored outline character representation normally does not represent space beyond the maximum horizontal and vertical boundaries of the character (also referred to as "white space" or "side bearings"). Therefore, the stored character outline portion of a character font is often referred to as a black body (or "BB"). The width 380 of the character outline 320 is referred to as the black body width (or "BBW"). The width 392 of the space between the character origin 332 and the left side boundary of the character outline 320 is referred to as the left side bearing (or "LSB"). Similarly, the width 394 of the space between the concatenation point 334 and the right side boundary of the character outline 320 is referred to as the right side bearing (or "RSB"). Notice that the sum of the black body width 380, left side bearing 392 and right side bearing 394 is the same as the advance width 370.
A font generator is a program for transforming character outlines into bitmaps of the style and size required by an application. Font generators (also referred to as "rasterizers") typically operate by scaling a character outline to a requested size and can often expand or compress the characters that they generate to improve readability (referred to as "hinting" which is described in more detail below). Note that the left and right side bearings may have zero (0) or negative values. Note also that in characters used in Japanese and other Far Eastern languages, metrics analogous to advance width, left side bearing and right side bearing--namely, advance height (AH), top side bearing (TSB) and bottom side bearing (BSB) --may be used.
.sctn. 1.2.2.2 Rendering Text to Pixel Precision
In the following, known techniques for rendering text on an output device such as a display (or printer) is described in .sctn. 1.2.2.2.1. Then, an example illustrating round-off errors which may occur when using such known techniques is described in .sctn. 1.2.2.2.2.
.sctn. 1.2.2.2.1 Technique for Rendering Text
FIG. 4 is a high level diagram of processes that may be performed when an application requests that text be rendered on a display device. Basically, as will be described in more detail below, text may be rendered by: (i) loading a font and supplying it to a rasterizer; (ii) scaling the font outline based on the requested point size and the resolution of the display device; (iii) applying hints to the outline; (iv) filling the grid fitted outline with pixels to generate a raster bitmap; (v) scanning for dropouts (optional); (vi) caching the raster bitmap; and (vii) transferring the raster bitmap to the display device.
In the case of scaling fonts, the font unit coordinates used to define the position of points defining contours of a character outline are scaled to device specific pixel coordinates. That is, when the resolution of the em square is used to define a character outline, before that character can be displayed, it must be scaled to reflect the size, transformation and the characteristics of the output device on which it is to be rendered. The scaled outline describes the character outline in units that reflect the absolute unit of measurement used to measure pixels of the output device, rather than the relative system of measurement of font units per em. Specifically, with known techniques, values in the em square are converted to values in the pixel coordinate system in accordance with the following formula: ##EQU1##
where the character outline size is in font uints, and output device resolution is in pixels/inch.
The resolution of the output device may be specified by the number of dots or pixels per inch (dpi). For example, a VGA video monitor may be treated as a 96 dpi device, a laser printer may be treated as a 300 dpi device, an EGA video monitor may be treated as a 96 dpi device in the horizontal (X) direction, but a 72 dpi device in the vertical (Y) direction. The font units per em may (but need not) be chosen to be a power of two (2), such as 2048 (=211) for example.
FIG. 4 is a high level diagram of processes which may be performed by a known text rendering system. As shown in FIG. 4, an application process 410, such as a word processor or contact manager for example, may request that text be displayed and may specify a point size for the text. Although not shown in FIG. 4, the application process 410 may also request a font name, background and foreground colors and an absolute or relative screen location at which the text is to be rendered. The text and, if applicable, the point size, 412 are provided to a graphics display interface (or GDI) process (or more generally, a graphics display interface) 422. The GDI process 422 uses display information 424 (which may include such display resolution information as pixels per inch on the display) and character information 425 (which may be a character outline information which may be represented as points defining a sequence of contours such as lines and curves, advance width information and left side bearing information) to generate glyphs (or to access cached glyphs which have already been generated). Glyphs may include a bitmap of a scaled character outline (or a bounding box 360 containing black body 320 information), advance width 370 information, and left side bearing 392 information. Each of the bits of the bitmap may have associated red, green and blue luminous intensity values. The graphics display interface process 422 is described in more detail in .sctn. 1.2.2.2.1.1 below. The graphics display interface process 422, the display information 424, and the glyph cache 426 may be a part of, and effected by, an operating system, such as the Windows.RTM. CE or Windows NT.RTM. operating systems (from Microsoft Corporation of Redmond, Wash.) for example.
Glyphs (also referred to as digital font representations) 428' or 428, either from the glyph cache 426 or from the graphics display interface process 422, are then provided to a display driver management process (or more generally, a display driver manager) 435. The display driver management process 435 may be a part of a display (or video) driver 430. Typically, a display driver 430 may be software which permits a computer operating system to communicate with a particular video display. Basically, the display driver management process 435 may invoke a color palette selection process 438. These processes 435 and 438 serve to convert the character glyph information into the actual pixel intensity values. The display driver management process 435 receives, as input, glyphs and display information 424'. The display information 424' may include, for example, foreground/background color information, color palette information and pixel value format information.
The processed pixel values may then be forwarded as video frame part(s) 440 along with screen (and perhaps window) positioning information (e.g., from the application process 410 and/or operating system), to a display (video) adapter 450. A display adapter 450 may include electronic components that generate a video signal sent to the display 460. A frame buffer process 452 may be used to store the received video frame part(s) in a screen frame buffer 454 of the display adapter 450. Using the screen frame buffer 454 allows a single image of, e.g., a text string, to be generated from glyphs representing several different characters. The video frame(s) from the screen frame buffer 454 is then provided to a display adaptation process 453 which adapts the video for a particular display device. The display adaptation process 458 may also be effected by the display adapter 450.
Finally, the adapted video is presented to the display device 460, such as an LCD display for example, for rendering.
Having provided an overview of a text rendering system, the graphics display interface process 422 is now described in more detail in .sctn. 1.2.2.2.1.1 below. The processes which may be performed by the display driver are then described in more detail in .sctn. 1.2.2.2.1.2 below.
.sctn. 1.2.2.2.1.1 Graphics Display Interface
FIG. 5 illustrates processes that may be performed by a graphics display interface (or GDI) process 422, as well as data that may be used by the GDI process 422. As shown in FIG. 5, the GDI process 422 may include a glyph cache management process (or more generally, a glyph cache manager) 510 which accepts text, or more specifically, requests to display text, 412. The request may include the point size of the text. The glyph cache management process 510 forwards this request to the glyph cache 426. If the glyph cache 426 includes the glyph corresponding to the requested text character, it provides it for downstream processing. If, on the other hand, the glyph cache 426 does not have the glyph corresponding to the requested text character, it so informs the glyph cache management process 510 which, in turn, submits a request to generate the needed glyph to the type rasterization process (or more generally, a type rasterizer) 520. Basically, a type rasterization process 520 may be effected by hardware and/or software and converts a character outline (which may, recall, include points which define contours such as lines and curves based on mathematical formulas) into a raster (that is, a bitmapped) image. Each pixel of the bitmap image may have a color value and a brightness for example. A type rasterization process is described in .sctn. 1.2.2.2.1.1.1 below.
.sctn. 1.2.2.2.1.1.1 Rasterizer
To reiterate, the type rasterization process 520 basically transforms character outlines into bitmapped images. The scale of the bitmap may be based on the point size of the font and the resolution (e.g., pixels per inch) of the display device 460. The text, font, and point size information may be obtained from the application 410, while the resolution of the display device 460 may be obtained from a system configuration or display driver file or from monitor settings stored in memory by the operating system. The display information 424 may also include foreground/background color information, gamma values, color palette information and/or display adapter/display device pixel value format information. To reiterate, this information may be provided from the graphics display interface 422 in response to a request from the application process 410. If, however, the background of the text requested is to be transparent (as opposed to opaque), the background color information is what is being rendered on the display (such as a bitmap image or other text for example) and is provided from the display device 460 or the video frame buffer 454.
Basically, the rasterization process may include two (2) or three (3) sub-steps or sub-processes. First, the character outline is scaled using a scaling process (or more generally, a scaling facility) 522. This process is described below. Next, the scaled image generated by the scaling process 522 may be placed on a grid and have portions extended or shrunk using a hinting process (or more generally, a hinting facility) 526. This process is also described below. Then, an outline fill process (or more generally, an outline fill facility) 528 is used to fill the grid-fitted outline to generate a raster bitmap. This process is also described below.
When scaling fonts in conventional systems such as TrueType.TM. from Microsoft Corporation of Redmond, Wash., the font unit coordinates used to define the position of points defining contours of a character outline were scaled to device specific pixel coordinates. That is, since the resolution of the em square was used to define a character outline, before that character could be displayed, it was scaled to reflect the size, transformation and the characteristics of the output device on which it was to be rendered. Recall that the scaled outline describes the character outline in units that reflect the absolute unit of measurement used to measure pixels of the output device, rather than the relative system of measurement of font units per em. Thus, recall that values in the em square were converted to values in the pixel coordinate system in accordance with the following formula: ##EQU2##
where the character outline size is in font uints, and output device resolution is in pixels/inch.
Recall that the resolution of an output device may be specified by the number of dots or pixels per inch (dpi).
The purpose of hinting (also referred to as "instructing a glyph") is to ensure that critical characteristics of the original font design are preserved when the glyph is rendered at different sizes and on different devices. Consistent stem weights, consistent "color" (that is, in this context, the balance of black and white on a page or screen), even spacing, and avoiding pixel dropout are common goals of hinting. In the past, uninstructed, or unhinted, fonts would generally produce good quality results at sufficiently high resolutions and point sizes. However, for many fonts, legibility may become compromised at smaller point sizes on lower resolution displays. For example, at low resolutions, with few pixels available to describe the character shapes, features such as stem weights, crossbar widths and serif details can become irregular, or inconsistent, or even missed completely.
Basically, hinting may involve "grid placement" and "grid fitting". Grid placement is used to align a scaled character within a grid, that is used by a subsequent outline fill process 528, in a manner intended to optimize the accurate display of the character using the available sub-pixel elements. Grid fitting involves distorting character outlines so that the character better conforms to the shape of the grid. Therefore, hinting can change the advance width 370 of a font. Grid fitting ensures that certain features of the glyphs are regularized. Since the outlines are usually distorted at only a specified number of smaller sizes, the contours of the fonts at high resolutions usually remain unchanged and undistorted.
In grid placement, sub-pixel element boundaries may be treated as boundaries along which characters can, and should, be aligned or boundaries to which the outline of a character should be adjusted.
Other known hinting instructions may also be carried out on the scaled character outline. FIG. 6 illustrates an unhinted letter "w". Notice that the font is asymmetric and that some of the details of the slanted line are lost. FIG. 7 illustrates the letter "w" after hinting instructions have been applied. In this case, the advance width 370 has been enlarged by two (2) pixels.
In an implementation of anti-aliased text for TrueType.TM. fonts supported in Windows NT.TM. 4, the hinted image 627 is overscaled four (4) times in both the X and Y directions. The image is then sampled. More specifically, for every physical pixel, which is represented by 4.times.4 portion of the grid in an overscaled image, the blend factor alpha for that pixel is determined by simply counting the squares whose centers lie within the glyph outline and dividing the result by 16. As a result, foreground/background blend factor alpha is expressed as k/16 and is computed for every pixel. This whole process is also called standard anti-aliasing filtering. Unfortunately, however, such standard anti-aliasing tends to blur the image. Similar implementation exists in Windows 95 and Windows 98, and the only difference is that the image is overscaled two (2) times in both X and Y, so that alpha for every pixel is expressed as k/4, where k is a number of squares within the glyph outline.
The outline fill process 528 basically determines whether the center of each pixel is enclosed within the character outline 320. If the center of a pixel is enclosed within the character outline 320, that pixel is turned ON. Otherwise, the pixel is left OFF. The problem of "pixel dropout" may occur whenever a connected region of a glyph interior contains two (2) ON pixels that cannot be connected by a straight line that passes through only those ON pixels. Pixel dropout may be overcome by looking at an imaginary line segment connected two (2) adjacent pixel centers, determining whether the line segment is intersected by both an on-transition contour and off-transition contour, determining whether the two (2) contour lines continue in both directions to cut other line segments between adjacent pixel centers and, if so, turning pixels ON.
The rasterized glyphs are then cached in glyph cache 426. Caching glyphs is useful. More specifically, since most Latin fonts have only about 200 characters, a reasonably sized cache makes the speed of the rasterizer almost meaningless. This is because the rasterizer runs once, for example when a new font or point size is selected. Then, the bitmaps are transferred out of the glyph cache 426 as needed.
The scaling process 522 of the known system just described may introduce certain rounding errors. Constraints are enforced by (i) scaling the size and positioning information included in a character font as a function of the point size and device resolution as just described above, and (ii) then rounding the size and positioning values to integer multiples of the pixel size used in the particular display device. Using pixel size units as the minimum (or "atomic") distance unit produces what is called "pixel precision" since the values are accurate to the size of one (1) pixel.
Rounding size and positioning values of character fonts to pixel precision introduces changes, or errors, into displayed images. Each of these errors may be up to 1/2 a pixel in size (assuming that values less than 1/2 a pixel are rounded down and values greater than or equal to 1/2 a pixel are rounded up). Thus, the overall width of a character may be less precise than desired since the character's advance width 370 is (may be) rounded. In addition, the positioning of a character's black body within the total horizontal space allocated to that character may be sub-optimal since the left side bearing 392 is (may be) rounded. At small point sizes, the changes introduced by rounding using pixel precision can be significant.
.sctn. 1.2.2.3 Rendering Text to Sub-Pixel Precision: Resolution Enhancement
FIG. 8 is a high level diagram of processes that may be performed to enhance the resolution of characters, such as fonts, as well as data accepted by or generated by such processes. As shown, the processes may act on an analytic image 412/425, such as contours, and foreground and background colors of a character.
An overscaling or oversampling process (or more generally, overscaling facility) 522'/710' may accept analytic character information, such as contours for example, and a scale factor or grid 820 and overscale and/or oversample the analytic character information. In this context, given an analytic character outline arranged on a grid defined by a coordinate system, overscaling means stretching the analytic character outline while leaving the coordinate system unchanged, while oversampling means compressing the grid defined by the coordinate system while leaving the analytic character outline unchanged. In the first case, in which the analytic character outline is overscaled, an overscaled analytic image 805 is generated. The overscaled analytic image 805 may then be sampled by sampling process (or more generally, a sampler) 806 to generate ultra-resolution digital image information 810. In the second case, in which the analytic character outline (which may have been overscaled) is oversampled, the ultra-resolution digital image information 810 is generated directly. The ultra-resolution image 810 has a higher resolution than the display 460 upon which the character is to be rendered. In one example, if the display is a RGB striped LCD monitor for example, the ultra-resolution image may have a resolution corresponding to the sub-pixel component resolution of the display, or an integer multiple thereof. For example, if a vertically striped RGB LCD monitor is to be used, the ultra-resolution image 810 may have a pixel resolution in the Y direction and a 1/3 (or 1/3N, where N is an integer) pixel resolution in the X direction. If, on the other hand, a horizontally striped RGB LCD monitor is to be used, the ultra-resolution image 810 may have a pixel resolution in the X direction and a 1/3 (or 1/3N) pixel resolution in the Y direction.
The optional hinting process (or more generally, a hinting facility) 526' may apply hinting instructions to the overscaled analytic image 805. In one embodiment, the overscaling and/or oversampling process 522' overscales the analytic image 415/425 by a factor of an arbitrarily large number N (e.g., N=sixteen (16)) in the X direction and does not scale the analytic image 415/425 in the Y direction. By doing this, in many cases, the hinting instructions of the optional hinting process 526' will not cause problems in the X direction (e.g., change the advance width of the character), which might otherwise occur. In this embodiment, the downscaling process 807 actually downscales the hinted image by Z/N, where Z is the number of samples per sub-pixel element desired (e.g., Z/N=6/16). The resulting scaled analytic image 808 is then sampled by sampling process 806. Consequently, the resulting ultra-resolution digital image information 810 is oversampled by Z (e.g., six (6)) in the X direction. That is, there may be Z (e.g., six (6)) samples per pixel or Z/3 (e.g., two (2)) samples per sub-pixel component.
FIG. 9 illustrates an example of the operation of an exemplary overscaling/oversampling process 522'/710' used in the case of a vertically striped LCD monitor. First, font vector graphics (e.g., the character outline), point size and display resolution are accepted. This information is denoted 412/425/910 in FIG. 9. The font vector graphics (e.g., the character outline) 412/425/910 is rasterized based on the point size, display resolution and the overscale factors (or oversample rate). As shown in the example of FIG. 9, the Y coordinate values of the character outline (in units of font units) are scaled as shown in 920 and rounded to the nearest integer pixel value. On the other hand, the X coordinate values of the character outline (in units of font units) are overscaled as shown in 930 and rounded to the nearest integer scan conversion source sample (e.g., pixel sub-component) value. Further, horizontal (or X) glyph metrics, such as advance width, left side bearing and right side bearing, are also overscaled as shown in 930 and rounded to the nearest integer scan conversion source sample (e.g., pixel sub-component) value. The resulting data 940 is the character outline in units of pixels in the Y direction and units of scan conversion source samples (e.g., pixel sub-components) in the X direction.
Then, referring back to FIG. 8, a process 830 for combining displaced (e.g., adjacent, spaces, or overlapping) samples (or more generally, a filtering facility) of the ultra-resolution image 524' can be used to generate another ultra-resolution image 840 (or an image with sub-pixel information) which may then be cached into cache storage 880 by the optional caching process 870. Each sample of the ultra-resolution image 840 may be based on the same number or differing numbers of samples from the ultra-resolution image 810. The cached character information 870 may then be accessed by a compositing process 850 which uses the foreground and background color information 424'.
Having described processes which may be used to perform an exemplary resolution enhancement function, special formatting concerns are now addressed in .sctn. 1.2.2.3.1 below.
.sctn. 1.2.2.3.1 Maintaining Formatting
Recall that the optional hinting process 526' may be applied to the overscaled analytic image 805. FIG. 10 illustrates a resolution enhanced character, with hinting, if any, applied. Comparing FIG. 10 with FIG. 7, notice that the black body width 380 has changed. Although not shown, the advance width 370, denoted by point "20", has also changed. (Recall FIG. 9.) Character widths are important when laying out text--changing the width of a character for a given font and point size may disturb formatting in existing documents. Changing the width of a character for a given font and point size may also disturb text within "dialog boxes", such that a message or words in a dialog box, or button, may shrink, become off-center, or extend outside of the dialog box or button. This presents an important challenge. Resolution enhancement techniques that have been developed, or that are being developed, promise significant advances in the readability of characters rendered on patterned displays. Such resolution enhancement techniques are particularly important when fonts with small point sizes are used, which is becoming more and more common given the increased use and acceptance of smaller, compact, untethered computing devices or information appliances. However, the formatting of resolution enhanced text should be backwards compatible. That is, text that was formatted with standard typographic techniques should maintain its formatting when enhanced resolution typographic techniques are used.
.sctn. 1.2.3 Unmet Needs
In view of the errors introduced when rounding character values, line drawings, or high resolution or analytic graphics to pixel precision as introduced above, methods and apparatus to increase the legibility and perceived quality of text are needed. Such methods and apparatus should maintain the formatting of text generated using standard typographic techniques and/or existing text processing applications. Such methods and apparatus should also minimize or eliminate any readily perceptible or annoying variations in character spacing and character line weight.